This invention relates to improvements in seismic sources for use in seismic surveys for detecting, locating, and delineating shallow target objects for engineering and geotechnical purposes.
In particular, the electrodynamic vibrator sources disclosed in U.S. Pat. No. 6,119,804 and in U.S. Pat. No. 6,488,117 have been effective in generating wide bandwidth controlled seismic waves defined as horizontally polarized shear (SH) waves and compressional (P) waves, respectively, at frequencies in the range 30-700 Hz, and higher, by their respective modes of operation and coupling to the ground surface. These sources have been used in combination with conventional geophysical exploration geophone sensors to detect, locate, and delineate abnormal soil conditions surrounding underground sewer pipes; detect and map shallow abandoned coal mines, image shallow soil-bedrock interfaces and paleochannels; and delineate fractures in shallow rock formations. In general, limitations caused by anomalies in the frequency response of conventional seismic geophones have prevented effective application of the vibrator sources at frequencies higher than 700 Hz as a means of detecting smaller underground targets and in resolving smaller details of subsurface target structures.
Conventional seismic field surveys employing relatively large one-dimensional linear or two-dimensional surface area layouts of geophone sensors and the placement of the seismic source at many positions along or within such layouts, while effective for deep geophysical exploration purposes, are technically inappropriate for shallow seismic reflections surveys in near-surface soil formations. The limitations largely relate to the nature of the shallow and localized underground targets of interest, such as pipes, cavities, and foundations, and the fact that conventional seismic data acquisition and analysis is labor intensive and expensive. Therefore, a more appropriate field methodology utilizing compact and semi-mobile equipment deployment and minimum data processing and analysis is needed to make near-surface high-resolution seismic surveys more productive and cost effective.
Attempts to obtain high resolution seismic survey results indicates that sensor devices such as accelerometers appropriately designed for seismic applications could provide improved frequency response above 700 Hz with the potential for uniform high frequency response up to about 2,000 Hz, and higher, consistent with the high-frequency operating capabilities of the above mentioned seismic sources. Additionally, attempts to extend other aspects of conventional seismic exploration techniques to higher frequency operation, such as variations in the placement and layout of the source and sensors on the ground, suggests that changes in the standard field operating techniques could also improve the overall success of shallow high-resolution seismic applications.
In contrast with using a scaled down version of conventional seismic survey methodology using relatively large geophone layouts and multiple source positions to acquire subsurface reflection data, a more compact alternative technique is one in which the vibrator source and a single compact sensor array are closely spaced and step-wise scanned over the survey area as a source-sensor pair. This compact source and sensor assembly is essentially a vertically down-going transmission and a vertically up-going reflection survey system. Therefore, since the field of view of such a system is intentionally restricted to near vertical seismic propagation paths, the source and sensor components can be made directional and primarily responsive to subsurface targets located essentially directly below the source-sensor pair on the surface. The seismic vibrator sources mentioned above are already capable of directional radiation of either SH waves or P waves, as a result of the relatively large area of the ground contact base plate as measured in wavelengths at the operating frequency. In contrast, a single accelerometer sensor operates as a point detector responsive to reflections from a wide field of view. Thus, to have a directional field of view, the sensor component also requires a relative large ground contact area, comparable to that of the vibrator source, to provide a useful companion component in the source-sensor pair.
Conceptually, the desired sensor directivity could be achieved by mounting a single accelerometer sensor on a base plate of approximately the same size as the base plate of the vibrator. However, in this arrangement, the sensor base plate must be rigid and, hence, heavy and properly damped to avoid undesirable spurious mechanical vibration resonance modes. Further, such an arrangement is a relatively limited technical approach which will result in reduced sensitivity to ground vibrations because of its large dynamic inertial mass.
Alternately, a cluster of individual accelerometer sensors arrayed in the size and shape of the aforementioned base plate may be used as a much more effective sensor system. An array of point transducers, operating either as sources or detectors, can serve as a beam-forming radiating or receiving aperture provided that the elements of the array are spaced less than one-half wavelength apart at the highest frequency of interest. Such an array has a number of important advantages when operating as a directional sensor. Such advantages include, but are not limited to:                a. Elimination of spurious mechanical vibrations associated with plate-type apertures;        b. An array of independent sensors mounted on a flexible panel capable of conforming with and coupling to an irregular or uneven ground surface;        c. An array having minimum required weight and bulk;        d. A multiple-channel array of sensors, in contrast with a single planar base plate, which may be used to implement advanced array operating techniques such as directional beam steering and focused response at selectable reflection depths;        e. Additive operation of independent sensor elements to enhance the response to coherent reflection signals while suppressing incoherent noise;        f. Sufficiently high directivity in typical near-surface soil media to detect high-resolution reflections from small localized targets such as utility distribution pipes and conduits.        
Such advantages, together with novel methods for reducing the required number of active sensor elements needed to implement large directional arrays and novel isolation mounting of the individual sensor elements and sub-arrays may be combined to produce a new and effective directional seismic sensor array as described in further detail below.
The directional seismic sensor system described hereinbelow offers a significant improvement over conventional seismic survey sensor technology for use in near-surface high-resolution seismic reflection imaging applications.